Two-piece golf ball

ABSTRACT

A two-piece golf ball includes a solid core and a cover. A difference between the maximum JIS-C hardness and the minimum JIS-C hardness of the solid core is in a range of less than 5. The cover is made from a material mainly containing a base resin composed of a Zn ion neutralized type ionomer resin and an Na ion neutralized type ionomer resin. The shore D hardness of the cover is in a range of less than 62. The thickness of the cover is in a range of 2.1 mm to 2.7 mm. The two-piece golf ball has a very high durability and is suitably used particularly as a training golf ball.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a two-piece golf ball includinga core made from a rubber-like elastic body and a cover formed to coverthe core, and particularly to a two-piece golf ball having a very highdurability and being suitably used particularly as a training golf ball.

[0002] In general, two-piece golf balls have been widely used forgolfers because of its advantage that they are excellent in durability.

[0003] With respect to these two-piece golf balls, the feeling ofhitting the ball has been also significantly improved as compared withconventional ones, and at present, a large number of two-piece golfballs acceptable to players from this viewpoint have been proposed.

[0004] By the way, since many players properly use golf balls each forgame or training purpose, a requirement for golf balls for game purposeand a requirement for golf balls for training purpose are slightlydifferent from each other.

[0005] For example, in the case of golf balls for game purpose, theflying performance of the ball is generally most important.

[0006] On the contrary, in the case of golf balls for training purpose,the durability against repeated hitting of the ball is most important.Accordingly, as training golf balls, there have been generally usedone-piece golf balls.

[0007] Recently, however, player's requirements for golf balls have cometo be variously broadened, and even training golf balls have beenrequired to give players a feeling of hitting the ball similar to afeeling of hitting a golf ball for game purpose, that is, required toallow players to practice playing golf with a feeling of game.

[0008] On the other hand, unless training golf balls not only meet theabove-described player's requirement but also have a long exchangecycle, that is, a good durability of the ball, such golf balls aredifficult to be adopted by owners of training clubs from the economicalviewpoint.

SUMMARY OF THE INVENTION

[0009] In view of the foregoing, the present invention has been made,and an object of the present invention is to provide a two-piece golfball having a very high durability and being suitably used particularlyas a training golf ball.

[0010] To achieve the above object, the present inventor has earnestlystudied and found that a two-piece golf ball including a solid core anda cover, which is characterized at the following points (1) to (4), iseffective to optimize respective characteristics of the core and thecover, and hence to prolong the durability and enhance a player'sfeeling, such as a feeling of hitting the ball comparable to thatobtained by a golf ball for game purpose by the synergistic effect ofthe optimized core and cover.

[0011] (1) A difference between the maximum JIS-C hardness and theminimum JIS-C hardness of the solid core is in a range of less than 5;

[0012] (2) the cover is made from a material mainly containing a baseresin composed of a Zn ion neutralized type ionomer resin and an Na ionneutralized type ionomer resin;

[0013] (3) the shore D hardness of the cover is in a range of less than62; and

[0014] (4) the thickness of the cover is in a range of 2.1 mm to 2.7 mm.

[0015] The present inventor has also found that such a two-piece golfball allows players to practice playing golf with a feeling of gamewhile prolonging the ball exchange cycle, and therefore, the golf ballis suitably used particularly in training clubs from the economicalviewpoint.

[0016] The present inventor has further found that the above two-piecegolf ball, characterized at the following point (5), is capable offurther improve the durability of the ball.

[0017] (5) The cover is made from a material mainly containing the baseresin to which an inorganic granular filler is added as a reinforcingmaterial in an amount of 11 parts by mass or more on the basis of 100parts by mass of the base resin.

[0018] The present inventor has accomplished the present invention onthe basis of the above-described knowledge.

[0019] Accordingly, the present inventor provides a two-piece golf ballincluding a solid core and a cover; wherein a difference between themaximum JIS-C hardness and the minimum JIS-C hardness of the solid coreis in a range of less than 5; the cover is made from a material mainlycontaining a base resin composed of a Zn ion neutralized type ionomerresin and an Na ion neutralized type ionomer resin; the shore D hardnessof the cover is in a range of less than 62; and the thickness of thecover is in a range of 2.1 mm to 2.7 mm.

[0020] In the above two-piece golf ball, preferably, the cover is madefrom a material mainly containing the base resin to which an inorganicgranular filler is added as a reinforcing material in an amount of 11parts by mass or more on the basis of 100 parts by mass of the baseresin.

[0021] The two-piece golf ball of the present invention thus configuredis very excellent in durability, and is suitably used as a training golfball.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] Hereinafter, a two-piece golf ball of the present invention willbe described in further detail. The two-piece golf ball of the presentinvention includes a core made from a rubber composition which mainlycontain a base rubber. As the base rubber, there may be used a knownnatural rubber or synthetic rubber, more specifically, polybutadiene,particularly, 1,4-cis-polybutadiene having at least 40% or more of acis-structure. The base rubber may further contain a natural rubber,polyisoprene rubber, styrene-butadiene rubber, in addition to theabove-described synthetic rubber, that is, polybutadiene, as needed.

[0023] According to the present invention, to give a desirableresilience to the core, an organic sulfur compound may be added to thebase rubber. As the organic sulfur compound, there may be usedthiophenol, thionaphthol, halogenated thiophenol, or a metal saltthereof. Specific examples of the organic sulfur compounds may includezinc salts of pentachlorothiophenol, pentafluorothiophenol,pentabromothiophenol, parachlorothiophenol, and pentachlorothiophenol;and polysulfides each having the sulfur number of 2 to 4, such asdiphenyl polysufide, dibenzil polysulfide, dibenzoyl polysulfide,dibenzothiazoyl polysulfide, and dithiobenzoyl polysulfide. Inparticular, a zinc salt of pentachlorothiophenol or diphenyldisulfidecan be preferably used.

[0024] According to the present invention, the added amount of theorganic sulfur compound is generally in a range of 0.05 part by mass ormore, preferably, 0.1 part by mass or more, more preferably, 0.2 part bymass or more on the basis of 100 parts by mass of the base rubber. Ifthe added amount is excessively small, the effect of improving theresilience cannot be expected. The upper limit of the added amount ofthe organic sulfur compound is generally in a range of 2.0 parts by massor less, preferably, 1.2 parts by mass or less, more preferably, 1.0part by mass or less on the basis of 100 parts by mass of the baserubber. If the added amount of the organic sulfur compound isexcessively large, the effect of improving the resilience (particularly,at the time of hitting the golf ball with the driver (or number one wood(W#1)) cannot be expected so much, and the core is excessively softenedto degrade the feeling of hitting the ball.

[0025] In addition to the above-described base rubber and the organicsulfur compound, known components may be added to the rubber compositionof the present invention, as needed. Examples of the known componentsmay include zinc salts of unsaturated carboxylic acids, organicperoxides, and antioxidants. The added amount of the above knowncomponent can be suitably adjusted.

[0026] The core can be produced from the above-described rubbercomposition for the core in accordance with a known process. One exampleof the process of producing the core includes the steps of kneading therubber composition by a Banbury mixer or kneader into a slug, puttingthe slug into a specific mold, and heating the slug at a heatingtemperature, at which the peak of the temperature rise at the centralportion caused by internal heat generation appears after an elapse of 20min or more since starting the heating, for 20 min or more andcontinuously heating the slug at a temperature higher than the aboveheating temperature.

[0027] The diameter of the core of the present invention may be, whilenot particularly limited thereto, generally in a range of 37.3 mm to38.8 mm, preferably, 37.5 mm to 38.5 mm.

[0028] With respect to the hardness of the core of the presentinvention, arbitrary points of the core are measured by a JIS-C typehardness tester, and a difference between the maximum value and theminimum value of the JIS-C hardnesses at the measured points of the coreis specified to be less than 5.

[0029] From the practical viewpoint, however, it is difficult tostrictly measure a JIS-C hardness at each point of the core anddetermine the maximum value and the minimum value of the measuredhardnesses, and therefore, according to the present invention, thedifference between the maximum JIS-C hardness and the minimum JIS-Chardness of the core is defined as follows:

[0030] The core to be measured is cut into halves, and JIS-C hardnessesat the following points (1) to (5) of one of the halves of the core aremeasured: (1) the core center, (2) the core surface, (3) a point apartfrom the core center by 5 mm, (4) a point apart from the core center by10 mm, and (5) a point apart from the core center by 15 mm. The highestand lowest values of the JIS-C hardnesses at the above measured pointsare specified as the maximum value and the minimum values, respectively.

[0031] With respect to the solid core of the present invention, thedifference between the maximum value and the minimum value of the JIS-Chardnesses at the above measured points is specified to be in a range ofless than 5. If the differential hardness is out of the above range, thedurability against cracking due to repeated hitting of the ball isdegraded.

[0032] In this case, the hardness of the core center may be generally ina range of 65 to 80, preferably, 68 to 77, more preferably, 70 to 74. Ifthe hardness of the core center is excessively low, the durability ofthe ball is degraded, and if the hardness of the core center isexcessively high, the feeling of hitting the ball may become hard.

[0033] The hardness of the core surface may be generally in a range of69 to 85; preferably, 72 to 82, more preferably, 74 to 79. If thehardness of the core surface is excessively low, the durability of theball is degraded, and if the hardness of the core surface is excessivelyhigh, the feeling of hitting the ball may become hard.

[0034] With respect to the solid core of the present invention, thecompressive deformation amount of the entire core, measured by applying,to the core, a load from an initial load of 10 kgf (98.07 N) to a finalload of 130 kgf (1274.91 N), may be generally in a range of 2.5 mm to5.0 mm, preferably, 2.8 mm to 4.0 mm, more preferably, 3.0 mm to 3.8 mm.If the compressive deformation amount is excessively small, the feelingof hitting the ball may become hard, and if the compressive deformationamount is excessively large, the durability against cracking may bedegraded and the feeling of hitting the ball may become excessivelysoft.

[0035] According to the golf ball of the present invention, the coreconfigured as described above is covered with a cover made from amaterial which mainly contains a base resin composed of a Zn ionneutralized type ionomer resin and an Na ion neutralized type ionomerresin for the purpose of improving the resilience and the durabilityagainst cracking.

[0036] Examples of the above Zn ion neutralized type ionomer resins mayinclude commercial ionomer resins (trade name: “Himilan” 1557, 1706, and1855, sold by Du Pont-Mitsui Polychemicals Co., Ltd.). Examples of theNa ion neutralized type ionomer resins may include commercial ionomerresins (trade name: “Surlyn” 1605, 1601, and 1707, sold by Du Pont DENEMOURS & COMPANY).

[0037] According to the present invention, the content of each of the Znion neutralized type ionomer resin and the Na ion neutralized typeionomer resin in the cover material may be generally in a range of 20parts by mass or more, preferably, 30 parts by mass or more, morepreferably, 40 parts by mass or more. If the content of each ionomerresin is excessively small, the effect of improving the resilience andthe durability against cracking may be degraded.

[0038] The cover material of the present invention, which mainlycontains the base resin composed of the above-described Zn ionneutralized type ionomer resin and Na ion neutralized type ionomerresin, may further contain an inorganic granular filler as a reinforcingmaterial.

[0039] To improve the dispersibility of the inorganic granular filler,while not exclusively, the average particle size thereof may be set in arange of 0.01 to 100 μm.

[0040] As the inorganic granular filler of a reinforcing material, theremay be used a barium sulfate.

[0041] The added amount of the inorganic granular filler may begenerally in a range of 11 parts by mass or more, preferably, 15 partsby mass or more on the basis of 100 parts by mass of the ionomer resinsas the base resin of the cover material. The addition of the inorganicgranular filler in the above range is effective to significantly improvethe durability without degradation of the flying performance. If theadded amount is excessively small, the reinforcing effect may beinsufficient. The upper limit of the added amount of the inorganicgranular filler may be generally in a range of 45 parts by mass or less,preferably, 35 parts by mass or less. If the added amount is excessivelylarge, the resilience may be degraded.

[0042] The two-piece golf ball of the present invention can be producedby covering the above-described core with the above-described covermaterial in accordance with, for example, a known injection moldingprocess or a known compression molding process. The former process isperformed by putting a previously prepared solid core in a mold, andinjection-molding a cover material on the surface of the solid core. Thelatter process is performed by previously preparing a pair of cup halveswith a specific cover material, covering a solid core with the cuphalves, and pressing the solid core covered with the cup halves. In eachprocess, the molding condition may be suitably changed.

[0043] According to the present invention, the thickness of the cover isrequired to be adjusted so as to obtain a synergistic effect with theimproved core of the present invention. To be more specific, thethickness of the cover may be in a range of 2.1 mm or more, preferably,2.2 mm or more. The upper limit of the thickness of the cover may be 2.7mm or less, preferably, 2.5 mm or less, more preferably, 2.4 mm or less.If the thickness of the cover is excessively small, the durabilityagainst cracking is significantly degraded, while if the thickness ofthe cover is excessively large, the resilience is reduced to degrade theflying performance of the ball.

[0044] The two-piece golf ball thus produced has a very high durabilityand gives players a feeling of hitting the ball comparable to thatobtained by a golf ball for game purpose, thereby allowing players topractice playing golf with a feeling of game while prolonging the ballexchange cycle because of its high durability. The golf ball of thepresent invention, therefore, is suitably used particularly in trainingclubs from the economical viewpoint.

[0045] The two-piece golf ball of the present invention can be producedwith its diameter and weight adjusted in accordance with the Rules ofGolf. That is to say, the golf ball having a diameter of 42.67 mm ormore and a weight of 45.93 g or less can be produced.

EXAMPLES

[0046] The present invention will be more clearly understood by way of,while not limited thereto, the following Examples and ComparativeExamples.

Examples 1 to 3 and Comparative Examples 1 to 7

[0047] Two-piece golf balls were produced using core materials shown inTable 1 and cover materials shown in Table 2 in accordance with a knownprocess.

[0048] Each of the two-piece golf balls thus produced was then subjectedto the following tests. The main materials used for producing thetwo-piece golf balls and evaluation methods used for the tests are asfollows:

[0049] [BR]: polybutadiene, sold by Japan Synthetic Rubber Co., Ltd.

[0050] Peroxide 1: dicumyl peroxide (trade name: “Percumyl D”, sold byNOF CORPORATION

[0051] Peroxide 2: 1,1-bis(t-butylperoxy)3,3,5-trimethyl cyclohexane(trade name: “Perhexa 3M-40”, sold by NOF CORPORATION)

[0052] Antioxidant: “NOCRAC NS-6” (trade name, sold by Ouchi-shinkoChemical Industrial Co., Ltd.)

[0053] Zinc salt of pentachlorothiophenol: “Renacit IV” (trade name,sold by Bayer LTD.)

[0054] [Himilan]: ionomer resin sold by Du Pont-Mitsui PolychemicalsCo., Ltd.

[0055] [Surlyn]: ionomer resin sold by Du Pont DE NEMOURS & COMPANY)

[0056] [Nucrel]: ethylene-metacryl acid-acrylic ester three componentcopolymer sold by Du Pont-Mitsui Polychemicals Co., Ltd.

[0057] Barium sulfate 1: “BARICO #100” (trade name, sold by Hakusui TecCo., Ltd.)

[0058] Barium Sulfate 2: “Barium Sulfate 300” (trade name, sold by SakaiChemical Industry Co., Ltd.)

[0059] Flying Performance

[0060] Each golf ball was hit at a specific point by a hitting robotwith its head speed set to a low head speed of 45 m/sec. The flyingdistance of the ball was measured and evaluated on the basis of thefollowing criterion:

[0061] G: total flying distance=220 m or more

[0062] B: total flying distance=215 m or less

[0063] Durability Against Cracking

[0064] Each golf ball was repeatedly hit at random points by the hittingrobot with its head speed set to 45 m/s, and the number of initiallyappeared cracks of the ball was evaluated. The number of initiallyappeared cracks was expressed by an index based on the number ofinitially appeared cracks of the golf ball in Example 3, which was takenas 100. In addition, the hitting of each golf ball was repeated by tentimes, and the number of earliest appeared cracks was taken as thenumber of initially appeared cracks to be evaluated.

[0065] G: index=100 or more

[0066] B: index=90 or less TABLE 1 Example Comparative Example 1 2 3 1 23 4 5 6 Core BR 01 50 50 50 50 50 50 50 50 50 composition BR 11 50 50 5050 50 50 50 50 50 (parts Zinc acrylate 33.4 31.6 30.7 30.7 31.6 31.627.5 30.7 30.7 by mass) Peroxide 1 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6Peroxide 2 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 Antioxidant 0.2 0.2 0.20.2 0.2 0.2 0 0.2 0.2 Barium sulfate 1 5.5 6.4 6.5 18.1 26.8 15.0 9.018.1 18.1 Zinc oxide 5 5 5 5 5 5 5 5 5 Zinc salt of 1 1 1 1 1 1 1 1 1pentachlorothiophenol Zinc stearate 0 0 0 0 0 0 5 0 0 VulcanizingPrimary Vulcanizing 145 145 145 145 145 145 157 145 145 condition stagetemperature (° C.) Vulcanizing time 30 30 30 30 30 30 15 30 30 (min)Secondary Vulcanizing 170 170 170 170 170 170 170 170 stage temperature(° C.) Vulcanizing time 10 10 10 10 10 10 10 10 (min) JIS-C Surface 7976 74 74 76 76 80 74 74 hardness Point apart from center by 76 74 72 7274 74 75 72 72 15 mm Point apart from center by 76 73 71 71 73 73 68 7171 10 mm Point apart from center by 74 71 70 70 71 71 65 70 70 5 mmCenter 74 71 70 70 71 71 63 70 70 Difference in hardness 5 5 4 4 5 5 174 4 (maximum value − minimum value)

[0067] TABLE 2 Example Comparative Example 1 2 3 1 2 3 4 5 6 7* CoreOuter diameter (mm) 38.3 38.3 37.9 38.3 35.7 39.5 38.3 38.3 38.3 —Compressive deformation 3.1 3.5 3.7 3.7 3.5 3.5 3.7 3.7 3.7 — amount(mm) Cover com- Himilan 1706 (Zn) 50 100 position Himilan 1557 (Zn) 3535 35 50 35 (parts Himilan 1855 (Zn) 15 15 15 35 15 by mass) Himilan1707 (Na) 100 Himilan 1605 (Na) 30 30 30 50 30 Surlyn 8120 (Na) 20 20 2035 20 Himilan 1601 (Na) 50 Nucrel AN4311 30 Barium sulfate 2 22 22 22 22Titanium oxide 5 5 5 5 5 5 5 5 5 Cover Thickness (mm) 2.2 2.2 2.4 2.23.5 1.6 2.2 2.2 2.2 Shore D hardness 61 61 61 63 43 60 61 60 62 BallStructure 2P 2P 2P 2P 2P 2P 2P 2P 2P 1P Outer diameter (mm) 42.7 42.742.7 42.7 42.7 42.7 42.7 42.7 42.7 42.6 Weight (g) 45.3 45.3 45.3 45.345.3 45.3 45.3 45.3 45.3 45.5 Performance W#1 Carry (m) 216.0 215.5215.1 218.2 205.1 214.0 213.9 213.8 213.4 197.2 of Ball HS45 Total (m)225.5 225.3 224.1 228.0 214.6 223.8 225.2 222.2 221.2 208.4 Evaluationof flying distance G G G G B G G G G B Random Durability hitting againstG G G B G B B B B G with cracking HS45

[0068] As is apparent from the results shown in Table 2, each of thegolf balls in Examples 1 to 3 exhibits the flying performance comparableto that obtained by a general two-piece golf ball for game purpose, anda high durability against cracking.

[0069] On the contrary, the golf balls in Comparative Examples 1 to 7are inferior to the golf balls in Examples 1 to 3 in the followingpoints:

[0070] In Comparative Example 1, since the cover is excessively hard,the durability against cracking is poor.

[0071] In Comparative Example 2, since the cover is excessively thickand the resilience of the ball is low, the flying performance issignificantly poor.

[0072] In Comparative Example 3, since the cover is excessively thin,the durability against cracking is poor.

[0073] In Comparative Example 4, a difference between the maximum JIS-Chardness and the minimum JIS-C hardness of the core is excessivelylarge, the durability against cracking is poor.

[0074] In Comparative Example 5, since the cover is made from the singleZn ion neutralized type ionomer resin, the durability against crackingis poor.

[0075] In Comparative Example 6, since the cover is made from the singleNa ion neutralized type ionomer resin, the durability against crackingis poor.

[0076] Comparative Example 7, which is a commercial one-piece golf ball,is very inferior to the two-piece golf ball of the present invention interms of flying performance.

[0077] While the preferred embodiments of the present invention havebeen described using specific terms, such description is forillustrative purpose only, and it is to be understood that changes andvariations may be made without departing from the spirit or scope of thefollowing claims.

1. A two-piece golf ball comprising a solid core and a cover; wherein adifference between the maximum JIS-C hardness and the minimum JIS-Chardness of said solid core is in a range of less than 5; said cover ismade from a material mainly containing a base resin composed of a Zn ionneutralized type ionomer resin and an Na ion neutralized type ionomerresin; the shore D hardness of said cover is in a range of less than 62;and the thickness of said cover is in a range of 2.1 mm to 2.7 mm.
 2. Atwo-piece golf ball according to claim 1, wherein said cover is madefrom a material mainly containing said base resin to which an inorganicgranular filler is added as a reinforcing material in an amount of 11parts by mass or more on the basis of 100 parts by mass of said baseresin.